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Synth Secrets, Part 23: Formant Synthesis

March 28, 2011

VOWEL SOUND AS IN… F1 F2 F3

“ee” leap 270 2300 3000

“oo” loop 300 870 2250

“i” lip 400 2000 2550

“e” let 530 1850 2500

“u” lug 640 1200 2400

“a” lap 660 1700 2400

Finally, let’s take a look at how the FS1R imitates the frequency response of a harmonically rich signal (or noise) passed through a resonant low-pass analogue filter (see Figure 16, above right). Yes, yes… we’ve seen it all before, but bear with me one more time.

Surprisingly, we can reconstruct this frequency response using just two formants — one with a centre frequency of 0Hz and a Q of, say 0.1, and one with a centre frequency equal to the analogue filter’s Fc, and with a Q of, say, 10 (see Figure 17, right).

The result is remarkable. What’s more, we can make the formant-generated sound respond very similarly to the analogue case. To be specific, we can shift the perceived cutoff frequency by moving the centre frequency of the upper formant while narrowing the Q of the lower formant by an appropriate amount. Do this in real time, and you have a sweepable filter. Furthermore, we can increase and decrease the perceived resonance by increasing or decreasing the amplitude of the upper formant alone.